linux/Documentation/power/interface.txt
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   1Power Management Interface
   2
   3
   4The power management subsystem provides a unified sysfs interface to 
   5userspace, regardless of what architecture or platform one is
   6running. The interface exists in /sys/power/ directory (assuming sysfs
   7is mounted at /sys). 
   8
   9/sys/power/state controls system power state. Reading from this file
  10returns what states are supported, which is hard-coded to 'standby'
  11(Power-On Suspend), 'mem' (Suspend-to-RAM), and 'disk'
  12(Suspend-to-Disk). 
  13
  14Writing to this file one of those strings causes the system to
  15transition into that state. Please see the file
  16Documentation/power/states.txt for a description of each of those
  17states.
  18
  19
  20/sys/power/disk controls the operating mode of the suspend-to-disk
  21mechanism. Suspend-to-disk can be handled in several ways. We have a
  22few options for putting the system to sleep - using the platform driver
  23(e.g. ACPI or other suspend_ops), powering off the system or rebooting the
  24system (for testing).
  25
  26Additionally, /sys/power/disk can be used to turn on one of the two testing
  27modes of the suspend-to-disk mechanism: 'testproc' or 'test'.  If the
  28suspend-to-disk mechanism is in the 'testproc' mode, writing 'disk' to
  29/sys/power/state will cause the kernel to disable nonboot CPUs and freeze
  30tasks, wait for 5 seconds, unfreeze tasks and enable nonboot CPUs.  If it is
  31in the 'test' mode, writing 'disk' to /sys/power/state will cause the kernel
  32to disable nonboot CPUs and freeze tasks, shrink memory, suspend devices, wait
  33for 5 seconds, resume devices, unfreeze tasks and enable nonboot CPUs.  Then,
  34we are able to look in the log messages and work out, for example, which code
  35is being slow and which device drivers are misbehaving.
  36
  37Reading from this file will display all supported modes and the currently
  38selected one in brackets, for example
  39
  40        [shutdown] reboot test testproc
  41
  42Writing to this file will accept one of
  43
  44       'platform' (only if the platform supports it)
  45       'shutdown'
  46       'reboot'
  47       'testproc'
  48       'test'
  49
  50/sys/power/image_size controls the size of the image created by
  51the suspend-to-disk mechanism.  It can be written a string
  52representing a non-negative integer that will be used as an upper
  53limit of the image size, in bytes.  The suspend-to-disk mechanism will
  54do its best to ensure the image size will not exceed that number.  However,
  55if this turns out to be impossible, it will try to suspend anyway using the
  56smallest image possible.  In particular, if "0" is written to this file, the
  57suspend image will be as small as possible.
  58
  59Reading from this file will display the current image size limit, which
  60is set to 2/5 of available RAM by default.
  61
  62/sys/power/pm_trace controls the code which saves the last PM event point in
  63the RTC across reboots, so that you can debug a machine that just hangs
  64during suspend (or more commonly, during resume).  Namely, the RTC is only
  65used to save the last PM event point if this file contains '1'.  Initially it
  66contains '0' which may be changed to '1' by writing a string representing a
  67nonzero integer into it.
  68
  69To use this debugging feature you should attempt to suspend the machine, then
  70reboot it and run
  71
  72        dmesg -s 1000000 | grep 'hash matches'
  73
  74CAUTION: Using it will cause your machine's real-time (CMOS) clock to be
  75set to a random invalid time after a resume.
  76
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